The present invention concerns scratch-resistant coatings comprising a primer coat (P) adhering to a substrate to be coated and a scratch-resistant coat (S) adhering to the primer coat (P), whereby the primer coat (P) contains a cured epoxy resin obtainable from hardeners and multi-epoxy functional compounds, and whereby the scratch-resistant coat (S) is obtainable from hydrolysable silanes containing epoxy groups. The coatings according to the invention are particularly resistant to attack by water. The present invention further concerns products coated with these coatings.
In order to protect materials against damaging environmental influences, they are frequently provided with a coating. Siloxane-based coatings, also known as siloxane coats, that give the materials inter alia a scratch-resistant and chemical-resistant surface, have proven to be particularly suitable. Siloxane coats are therefore also known as scratch-resistant coats (S).
In order to ensure a good adhesion of the particularly hard scratch-resistant coat (S) to the generally softer substrates, an intermediate coat, also known as a primer coat (P) is frequently used between the substrate and the scratch-resistant coat (S). The purpose of this primer coat (P) is to compensate for the different expansion coefficients of the substrate and the scratch-resistant coat (S) and to bring about a good adhesion of the coats to the substrate.
The primer coat (P) is intended to prevent the protective scratch-resistant coat (S) from detaching from the substrate and from cracking under environmental influences such as heat, cold, dryness, moisture, chemicals, etc.
Long-term attack by hot water is particularly critical for the scratch-resistant coat (S).
Primer coats (P) and their use in combination with siloxane-based scratch-resistant coats (S) as well as products coated with them, particularly those made from polycarbonate, are described in the following documents: DE-A 2 917 837, DE-A 2 947 823, DE-A 2 950295, DE-A 3 014 581, DE-A 3 014 772, U.S. Pat. No. 4,243,720, DE-A 3 044 804. They describe primer coats (P) that are produced by thermal and/or radiation-induced polymerisation of acrylic monomers, preferably on polycarbonate.
The acrylic monomers such as methyl methacrylate, hydroxyethyl acrylate, ethyl methacrylate, glycidyl methacrylate, methacryloxypropyl trimethoxysilane, among others, which are generally used as a mixture, are applied to the substrate diluted with organic solvents or dispersed in water by flow coating, dip coating, spraying or other conventional painting techniques and then cured by thermal or radiation-induced reaction. In this way the primer coat (P) is formed.
Using a similar method of application the siloxane-based scratch-resistant coats (S) are then applied to the primer coats (P).
DE-A 3 121 385, EP-A 0 340 565 and U.S. Pat. No. 5,041,313 describe primer coats (P) in which the final polyacrylic polymers dissolved in organic solvents or as aqueous dispersions are used instead of the acrylate monomers that still have to be polymerised. The advantage of these systems lies in the fact that only drying is required, with no reaction step. These primer coats (P) are primarily described and used in combination with special siloxane systems.
The siloxane systems belonging to the prior art are substantially thermally curing systems that preferably crosslink by condensation reaction with formation of Si-O-Si linkages. Other crosslinking mechanisms can also proceed in parallel.
Such systems are described for example in U.S. Pat. Nos. 3,790,527, 3,865,755, 3,887,514, 4,243,720, 4,278,804, 4,680,232, 4,006,271, 4,476,281, DE-A 4 001 045, U.S. Pat. No. 4,122,743, 4,020,316, 3,917,535, 3,706,714, 3,407,087, 3,836,815, 2,914,427, 3,135,241, 3,134 777, 3,100,532, 3,151,350, 3,005,541, 3,014,411, 2,834,606, 2,947,879, 3,016,021, 2,914,427 and 4,338,361.
Particularly high, glass-like scratch resistances are achieved with coatings obtainable from hydrolysable silanes containing epoxy groups, as described in U.S. Pat. No. 5,134,191, DE-A 4 020 316 and WO 98/05198.
The primer coats (P) known from the prior art, when used as an intermediate coat between the particularly scratch-resistant siloxane coats obtainable from hydrolysable silanes containing epoxy groups and a substrate, particularly polycarbonate, are not suitable to prevent the scratch-resistant coating from detaching under long-term attack by hot water (more than 4 days).